A watermark in an audio or video signal can be detected at receiving or decoder side using correlation, as described for example in WO 2007/031423, WO 97/33391 (U.S. Pat. No. 6,584,138 B1) or U.S. Pat. No. 6,061,793.
Many watermarking systems make use of correlation for calculating a detection metric, which means that several pseudo-random sequences or reference patterns are generated, or read from a memory, at encoder side and one or more of them are embedded inside the content (e.g. an audio or video signal), dependent on the message to be embedded. Normally, the reference patterns are orthogonal to each other. The same pseudo-random sequences are generated, or read from a memory, at decoder side. Frequency transform may be used to en-ode and decode the embedded message. To decode the embedded message, it is necessary to discover which pseudo-random sequence or sequences were embedded at encoder side. This is determined in these systems by correlating the known pseudo-random sequences with the possibly watermarked content, whereby the correlation may operate on a pre-processed version of the content, and that pre-processing may include inverse frequency transform, spectral shaping and/or whitening.
Each embedded reference pattern may represent a single bit of the embedded message, or two or more bits. A watermark detector decides, depending on the size of the correlation result values, whether or not a given pseudo-random sequence was embedded.